This invention relates to color imaging processors and, in particular, to an adaptive toner patch scheduler for implementing diagnostics for the processor's systems parameters and system fault conditions in a manner that minimizes the waste of toner materials without compromising image quality.
The xerographic imaging process is initiated by charging a charge retentive surface such as that of a photoconductive member to a uniform potential, and then exposing a light image of an original document onto the surface of the photoconductor, either directly or via a digital image driven laser. Exposing the charged photoconductor to light selectively discharges areas of the surface while allowing other areas to remain unchanged, thereby creating an electrostatic latent image of the document on the surface of the photoconductive member. A developer material is then brought into contact with the surface of the photoconductor, to transform the latent image into a visible reproduction. The developer typically includes toner particles with an electrical polarity the same as or opposite to the latent images on the photoconductive member. The polarity depends on the image profile. A blank copy sheet is brought into contact with the photoreceptor and the toner particles are transferred thereto by electrostatic charging the sheet. The images on the sheet are subsequently heated, thereby permanently affixing the reproduced image to the sheet. This results in a "hard copy" reproduction of the original document or image. The photoconductive member is then treated including cleaning to remove any charge and/or residual developing material from its surface to prepare it for subsequent imaging cycles.
Electrophotographic printers that operate by projecting a laser scan line onto a photoconductive surface are well known. In printers such as these, it is common to employ a Raster Output Scanner (ROS) as a source of signals to be imaged on the photographic member. The ROS provides a laser beam which switches on and off according to electronic image data associated with the image to be printed as the beam moves, or scans, across the charged photoreceptor. Laser diodes are typically used to generate the laser beam that is used to scan in a ROS system. The image data is driven in serial fashion to reproduce each line in the image. Modulation of the scanning beam is typically implemented by digitally controlling the output of the Light beam or a modulator associated with a continuous laser source. The latent electrostatic images on the photoreceptor may comprise either charged and/or discharged areas of the photoreceptor.
Electrophotographic laser printers, scanners, facsimile machines and similar document reproduction devices, must be able to maintain proper control over the systems of the image producing apparatus to assure high quality, hardcopy outputs. For example, the level of electrostatic charge on the photographic member must be maintained at a certain level to be able to attract the charged toner particles. The light beam must have the proper intensity in order to be able to discharge the photoreceptor. In addition, the toner particles must be at the proper concentration to ensure high print quality. As the printing machine continues to operate, changes in operating conditions will cause these parameters to vary from their initial values. For example, an increase in the humidity in environmental conditions around the corona discharge device used to generate the electrostatic charge on the photoreceptor will cause a decrease in the magnitude of the charge that is ultimately placed on the photoreceptor. Changes due to the variation in operative components of the machine also impact print quality. Thus, it is desirable to monitor the systems operating parameters of the machine to insure proper operation thereof.
One way to control the many parameters within machine that operate together to reproduce images is to use process control patches strategically positioned on the photoconductive or charge-retaining member of the apparatus. The control patches are usually generated by sending a known pattern of data to control the modulation of the light emitting elements in the writing head. Since the data patterns are known, the electrostatic charge that must be present on the surface of the photoreceptor to create it is also known. The control patches are deposited onto a small area of the photoreceptor between areas reserved for placement of the latent images, and the voltage levels across them are measured to provide an indication of their electrostatic charge. Feedback of information derived from the control patches allows for changes in one or more of the operating parameters, thereby enabling substantially constant image quality.
In existing xerographic print engines, sensor readings of toned control patches serve two purposes (1) to provide a basis for adjusting the appropriate system parameters such as corona charging and developer dispense rate to maintain print image quality and (2) to provide a basis for identifying and declaring system fault conditions such as photoreceptor voltage which is too high or too low. In other words, a determination of whether a voltage reading is outside of a target voltage range.
Prior art techniques for accomplishing control of system parameters require a large number of toner patch readings resulting in a significant waste of toner. Thus for system control, there is a strong desire to reduce the number of readings to the minimum required to adequately maintain the system parameters in order to conserve toner. However, the tracking of system fault conditions requires frequent toner patch readings to enable the machine to take corrective actions as soon as possible.
In a full color print engine that incorporates five separate xerographic imaging stations to create process color composite images (cyan, magenta, yellow, black, plus 1 `spot color` for extending the overall color gamut), the control of the Tone Reproduction Curve (TRC) for each station may use a toner coverage sensor such as an Extended Toner Area Coverage Sensor (ETACS) Infrared Densitometer (IRD) to sample three separate halftone patches developed on the photoreceptor in an Interpage Zone (IPZ) between customer images. Thus, the combined system requires a total of 15 separate patches for a five station processor to achieve the same function as a DocuCenter? printer that requires three patches for control of the TRC for black only.
To accommodate the need for extra patches, the above-described print engine uses three ETACS located across the photoreceptor to make fuller use of each separate interpage zone (IPZ). The IPZ length for this machine is enlarged to provide the space needed to rephase (i.e. align them together with each other) the multiple ROSS. This increased length permits three patches to be printed in-line for each ETACS. Both of these increases are limited by architectural constraints and, while quite helpful, still result in a 40% shortfall to the maximum sampling rate.